Wind turbines used in wind energy converters (also called wind generators) for the production of electric power make a valuable contribution towards obtaining energy from renewable sources. Wind energy converters can be onshore, i.e. land-based, or offshore, i.e. constructed in a coastal area. Onshore wind turbines are surpassed in performance by offshore turbines, since windy conditions generally prevail over the sea, while, depending on their location, land-based wind turbines may be subject to prolonged periods of low wind and therefore relatively lower power output. For this reason, increasing numbers of offshore turbines are being built, and these numbers are expected to increase in future. These developments, along with the larger size of the newer wind turbines, are leading to greater demands on serviceability and robustness. An offshore turbine, for obvious reasons, is more costly to service than a land-based or onshore turbine.
Several different types of wind turbines, or wind energy converters, are in use at present. Many use a main shaft with the hub and blades at one end and a gearbox at the other, but there are alternative designs. For example, the Vestas 3MW V90 uses a combined main bearing and gearbox, i.e. the gearbox is integral to the main bearing, and therefore does not have a main shaft. Another type of wind turbine does not have a gearbox, and the generator rotates with low speed. These wind turbines are called ‘direct drive generators’ and do not have a main shaft as such. These prior art wind turbines use rolling element bearings as main bearing to support the rotor and blades, and the main shaft if the wind turbine has one.
Typical service issues are the replacement of defective bearings, particularly the main bearing, which must support very high dynamic loads depending on wind turbine size and wind conditions. The load on the main bearing is primarily determined by the combined mass of blades, hub and main shaft, and by the wind speed. These high dynamic loads result in correspondingly large dynamic shaft deflections in wind turbines. The maximum rotational speed of a wind turbine is determined by the turbine size and therefore also the blade size. A larger turbine, with larger blades, gives a lower maximum rotational speed. For example, an existing 2.0 MW turbine has a maximum rotational speed of 19 rpm, while the maximum rotational speed of a 3.6 MW turbine is only 13 rpm. At these low speeds, the dynamic loads exerted on the main shaft and main bearing can be very high, especially in strong wind conditions, where resulting forces in the order of more than 2 MN (Mega Newton) exerted on the bearings are not infrequent. At start-up or shut-down, conditions become even more critical since the rotational speed is extremely low—for example less than 5 rpm—and friction between shaft and main bearing becomes greater. Evidently, the bearing load increases with increasing wind turbine size. The operating conditions combined with the issues that large rolling element bearings are very sensitive to material quality and require correct handling and lubrication make roller bearings or ball bearings more likely to fail during the required lifetime.
Since larger wind turbines are desired because of their overall better system economy, the lifetime and performance of the bearing is becoming a more critical aspect in wind turbine design, especially for offshore wind turbines. The conventional roller bearings are associated with a number of problems. Roller bearings must be machined to a high degree of precision, since any irregularity can quickly lead to material failure. Another major argument against the use of a roller bearing system in the main bearing of a wind turbine is the difficulty associated with its maintenance. Alternatives such as sliding or journal bearings, while being less susceptible than roller bearings to noise and vibration, are also unsuitable because of their intolerance to the edge loading that will occur due to the relatively flexible turbine structure. It is not possible to replace such a bearing, or a bearing part, without first disassembling the drive train. A suitably large external crane is required to lift the hub, blades, shaft and bearing off the turbine. The bearing can then be dismantled and replaced, and the components must then be lifted in place again for assembly. An external crane with the necessary lifting capacity presents a considerable additional expense, particularly for an offshore wind turbine, for which the external crane must be transported (in favourable weather conditions) by ship.
Such maintenance procedures in a wind energy converter are costly and time-consuming, particularly in an offshore location, as the skilled person will appreciate. Also, such maintenance can only be carried out during low-wind conditions. In an offshore location, however, conditions of low wind can be seldom. A turbine with a damaged bearing may then have to be furled for a long period of time until the wind drops, during which time the turbine cannot be used to generate electricity.
In brief, the current bearing systems do not satisfactorily fulfil the requirements of long lifetime and low service when used as the main bearing in a wind turbine.
It is therefore an object of the invention to provide an improved bearing for a wind turbine which avoids the problems mentioned above.